cycle.rb

class Cycle < ActiveRecord::Base
  belongs_to :user
  belongs_to :decision, polymorphic: true

  validates :user_id, presence: true
  validates :decision_id, presence: true
  validates :decision_type, presence: true
  validates :effective_at, presence: true
end

factories.rb

FactoryGirl.define do

  #...

  factory :rental do
    product
  end

  factory :cycle do
    user
    decision rental
    effective_at { Date.beginning_of_this_month }
  end
end

rental.rb

class Rental < ActiveRecord::Base
  has_one :cycle, as: :decision
  belongs_to :product
end

Removing line 11 in factories.rb stops the error.

Just in case someone else finds this someday: the issue was the association in the cycle factory needed to be declared like

factory :cycle do
  user
  association :decision, factory: rental # this is the change
  effective_at { Date.beginning_of_month }
end